We propose to examine by Cu and Fe EXAFS the cytochrome bo quinol oxidase of E. coli as an analog of the mammalian cytochrome c oxidase (aa3). Cytochrome bo has been cloned and expressed and site-directed mutated proteins have been characterized. Since cytochrome bo is one member of the cytochrome oxidase superfamily that does not contain the CuA site, Cu EXAFS will be able to directly probe the CuB coordination environment without interference from the other Cu site. We will take advantage of this characteristic to explore the structure of the dinuclear heme o-CuB site in various redox states, and in the presence of substrate O2, competitive inhibitors, and exogenous ligands.